1. Field of the Invention
This invention relates to production of films or sheets of good optical quality formed from one or more layers of plastic by application of at least a liquid mass such as a solution or a reaction mixture to a support in continuous movement relative to the applied material. More particularly, the invention relates to a process and device for said production.
The term "sheet", used below, will also designate both a thin film formed from a single layer of plastic and a sheet of undetermined length formed from one or more layers of plastic.
2. Background of the Prior Art
The publication of European Pat. No. 0,038,760 describes the production of a plastic sheet on a flexible, taut plastic sheet whose surface has been previously unwrinkled and made perfectly smooth, this support being supplied continuously from storage reels, and passing over a stationary table placed at the site where the liquid mass is applied.
The taut flexible strip must be kept constantly flat. For this purpose, in an embodiment of the process and device for using it, passage of the taut flexible strip is performed over a series of glass sheets forming a continuous surface and providing a constant, plane surface for the taut strip. This passage over a plane surface has drawbacks because it is done with considerable friction which creates electrostatic charges on the glass and taut strip. These electrostatic charges attract dust and other particles particularly between the glass surface and taut strip, which can cause impressions on the cast layer, hence optical defects, and also risks of tears in the taut strip. Dust or particles can also be attracted to the surface of the taut strip that is to be covered with the cast layer and consequently can be inlaid in said cast layer.
To limit the friction described above, a variant, described in this same publication, was proposed to replace the glass sheets forming the stationary surface with a multipilicity of metal wires stretched crosswise to the passage of the flexible strip. This system also has drawbacks: it does not offer a perfectly plane support and, further, dust can be stopped by the wires. This results in optical flaws in the form of lines on the formed film.